It's more than likely the rear pulley isn't centered 100% and creating loose and tight spots in the belt .You can slip the belt or loosen the belt , untighten then snug up the pulley bolts up so as no movement and use a dial type indicator or reference to take away high spots and center it that way. A bit of work but it may fix your issue .
And yea the coasting wuck wuck wuck is typical summer belt scenario.

 

Is this whoop whoop sound during deceleration and at any given speeds?

 

Just saw the post about 3/8" difference between loose and tight spots. What happens if you roll it fully 5 times and then 10 times and measure? There is no timing to the belt, and as it rotates through it will walk the pulleys - would be interested to see what measurements you get there. Happens with primary chains all the time.

Also, are you certain it's that much or are you rounding?

 

I just went threw this on my Ride, mechanics told me Adjust the drive belt on the Tight spot, Then another Mechanic (different shop) said Adjust the Drive belt on the loose spot.
My Manual said adjust drive belt on the loose spot, All models are different, Refer to your HD Service Manual.
I was in two shops for two different Reasons, and ask about the drive belt Adjustment Spot.
It did make more sense to adjust to the Tight spot, but not on my ride.

 

Once the drive line is hot it’s likely always there, it’s just that you only hear it when the throttle is closed.

 

You don’t say which bike you have so I’m not sure if your answer is relevant. I’ve looked in the service manual for my ‘19 Softail, and it makes no mention of “loose” or “tight” spots at all. Problem is, if you set the tension per the manual at 1/2”-9/16” cold, when the bike is hot the belt is tight as a banjo string. That can’t be right under any circumstance.

 

Make sure your alignment is good and set you belt to 5/8 at the tight spot with bike on the ground in upright position. That's what I did to get rid of the noise. My alignment was way out from the factory.

 

It does not matter what ride I have , The point was All models and year of ride are different, Refer to your HD Service Manual.

 

This is a no brainer. How often someone throws a tire on there?
Given the circumstances, me havin to do it with lil help, been there.
Dealer doin it? Ask them chck the belt tension.
RTFM? > laffs

 

I think I may have found the issue with the proper belt tension and the wah wah noise many Softail owners are having. Last night I was reading an old thread about belt tension and noisey belts. In the thread Hulkss mentioned that the the 2018+ Softail manuals states that the belt tension can be adjusted with the bike upright or on the jiffy stand with the saddlebags empty. He also mentioned that he tried the belt tension adjustment both ways upright and on the jiffy stand. He found that if he adjusted the belt to the proper spec with the bike on the jiffy stand and then checked the tension with the bike upright he found that the upright position gave a considerably tighter tension. So I decided to try for myself and wouldn't you know it Hulkss was right if I set my belt for 1/2 inch deflection with the bike on the jiffy stand and then checked the tension with the bike upright I found the tension now at about 1/4 inch deflection. So I don't know why Harley recommends setting the tension either on the jiffy stand or upright as you get a considerable difference in the tension reading. So I decided to adjust the belt with the bike in the upright position at 1/2 of deflection at the tighter spot in the belt. I took in for a ride tonight and wouldn't you know it the wah wah noise when coasting down between 30 20 mph is 90 prect gone. Now it was only about 75 " out. We will see when the temps get hotter but it seems more logical the set the belt tension in the upright position as that is the way you ride the bike. Setting tension on the jiffy stand seems to make you belt adjustment too tight there by causing more belt noise.